This 'Dark Night' is a very powerful piece of equipment and the NVRAM issue has been resolved, however, it presents significant risk (bricking) that requires careful adherence to the installation process.

22-February-2016. Stay away from K2.6 builds for this router. Chances are you don't have one with the original bootloader and even if you do, the K3.x-big-RT-N66U.trx build will work on the router regardless of bootloader.
Use the builds in the ftp://ftp.dd-wrt.com/betas/2016/02-19-2016-r29134/broadcom_K3X/ folder

15-May-2015 The command: nvram get bl_version can be issued to determine the CFE version if you have TELNET or SSH access. The Cellspot wiki contains links to many tools and articles to change the CFE. Please consider posting a link to a 64K CFE for future users if you have the knowledge \ resource to do so and delete this statement.

25-May-2013: Since BrainSlayer posted some experimental K3.X Images, the NVRAM Issue becomes almost obsolete. Please have a look at the Installation Instructions for further instructions.

9-Apr-2013: It seems more recently manufactured units come pre-loaded with the 64K v1.0.1.3 CFE and so are not compatible with standard DD-WRT builds. Flashing a standard DD-WRT build on these devices will result in a brick. If you don't know which CFE version your unit has when running the stock firmware,anyone wishing to flash DD-WRT on this device is advised to follow this guide: http://charleswilkinson.co.uk/2012/12/22/dd-wrt-on-the-asus-rt-n66u-with-64k-cfe/

1-Nov-2012: Fractal has indicated that the NVRAM is resolved and that 5Ghz is working.
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30-Oct-2012: RT-N66 CFE Thread Link to discussion implementing 64K: not sure that this is ready for prime time yet, however, if you try this and feel it is ready, UPDATE THIS STATEMENT and publish the procedure, limitations, and results.

The ASUS RT-N66U is a Dual Band N-Router (2.4GHz and 5GHz bands) with lots of RAM (256Mb) and FlashRAM (32Mb), so it is well equipped to run DD-WRT

The ASUS RT-N66U is in the GigaBit Routing class, meaning it can route at higher speeds than a 100Mbps WAN connection can handle. Performance details below.

If you are looking for less expensive routers, that are also well suited to run DD-WRT, you might want to check out the Asus RT-N16 Single Band N-Router (2.4GHz only) but still with lots of RAM (128Mb) and FlashRAM (32Mb). Even cheaper is the Asus RT-N10U Single Band N-Router with 32Mb RAM and 8Mb FlashRAM. See also ASUS Promo page

Reset button:
pressing the Reset button while power-up, will put this ASUS router in recovery mode.

That is: press and hold the reset button, keep pressing the reset button while plugging in the power, then keep holding the reset button pressed for about ten seconds until the power led is blinking slowly, then release the reset button. The router is now in recovery mode.

WPS button:
pressing the Red WPS button while power-up, will clear the NVRAM. All settings are stored in NVRAM of this ASUS router. Clearing the NVRAM is the most efficient way to reset the router to default settings.

That is: press and hold the WPS button, keep pressing the reset button while plugging in the power, then keep holding the WPS button pressed for about ten seconds, then release the WPS button. All settings have now been wiped, and default settings will be used.

25-May-2013: Since the experimental K3.X Broadcom Builds are out, you do not need to care what CFE you have. DD-WRT detects it automatically and extends 32KB NVRAM Version to 64KB. Which Image you have to flash, depends on your previous Image ( Stock, Merlin, DD-WRT K2.6 ).

For DD-WRT K2.6: It depends how much NVRAM ( 32KB / 64KB ) you currently have.For 32KB, flash 26138 mega.

If you are already on a custom 64K Build like Fractal or Kong with CFE 1.0.1.3, you need to flash 26138 mega nv64k. Since K2.6 Builds have an embedded check, you need to use those special nvXXk trailed images. On K3.X this check has been removed and you can simply upgrade from a previous K3.X to newer svn using dd-wrt.v24-YYYYY_NEWD-2_K3.x_mega.bin.

USB does not work in K2.6 builds, any attempt to use a USB device will cause a kernel panic see this trac ticket. If you want USB functionality with DD-WRT you'll need to use a K3.x build as the problem is fixed in K3.x builds, but won't however be fixed for K2.6 builds.

9-Apr-2013: It seems more recently manufactured units come pre-loaded with the 64K v1.0.1.3 CFE and so are not compatible with standard DD-WRT builds. Flashing a K2.6 DD-WRT build on these devices will result in a brick. As it is impossible to know which CFE version you unit has when running the stock firmware, anyone wishing to flash DD-WRT on this device is advised to follow this guide: http://charleswilkinson.co.uk/2012/12/22/dd-wrt-on-the-asus-rt-n66u-with-64k-cfe/ These instructions should be updated when an official build becomes available.

Wait for the router to re-boot on its own. This may easily take 10-15 minutes to happen, so just be patient. You want both ping response and web-gui response to know that the router is fully re-booted.

Clear settings (Clear NVRAM) select Factory Default settings. Wait for the router to re-boot on its own!!! May take 10 minutes or more

When completed wait until the router re-boots on its own. This takes at least 10 minutes.

Put router into recovery mode by holding down the reset button while powering up the router, wait until the power led is blinking slowly, then release the reset button.

Select Reset NVRAM defaults when it completes (should be quick), in the browser field, type in http://192.168.1.1/do.htm?cmd=nvram+commit. When done press enter. When complete select reboot and wait 5-10 minutes for the router to reboot and start building its layers.

Now you should be able to access via web gui 192.168.1.1 and it should be DD-WRT.

Then you want to release the plastic clips that holds the top and bottom parts together.

You want to "pry" the bottom case outward. The clips have the male part moulded into the bottom, the female part is moulded into the top cover.

For easier access next time, you can either snip off the "U" clips on the top cover, or round off the square edges on the male part of the clip with a nail file. Either solution makes it easier to get the case apart next time.

In addition to the NAS features of DD-WRT, you can use the extra space to install Optware.

11 Apr 2014: Please note that since USB & MMC support is not working on K2.6 builds, you cannot install Optware on K2.6 builds. USB & MCC are working on K3.x builds, however Optware support for K3.x builds is currently considered beta. Having said that, forum member kabadisha has successfully tested installation of Optware on K.3x build 23598 mega on this device using the following instructions: OTRW Take 2